1. Field of the Invention
The present invention relates to an image fixing apparatus for fixing a non-fixed toner image in an apparatus of an electrophotographic system, such as a copying machine, a laser beam printer, a facsimile equipment, or the like, and further relates to an image-fixing endless film for use in such an image fixing apparatus.
2. Background Art
Heat roll fixing is heretofore known as a method for fixing an image on a recording material in an image forming apparatus of an electrophotographic system. As shown in FIG. 1, the system uses a structure in which a heat source 12 such as a halogen heater is disposed inside a core 11 of at least one roll 10 of a pair of rolls and in which at least one layer of a heat-resistant elastic body 13 is provided on the core 11. In the system, the layer of the elastic body 13 excellent in elasticity is provided on the at least one roll 10 and a higher nipping pressure is applied to the roll 10 in order to widen the area of a nip portion formed for applying solderless contact on a non-fixed toner image 5. However, if the roll excellent in elasticity was used under a high temperature while a high nipping pressure was applied to the roll, there was a problem in durability that the elasticity of the roll deteriorated. Further, in the case where such a pair of rolls as shown in the system were used, it was necessary to provide a drive/driven control mechanism (not shown) on at least one end portion of the rolls in order to make the rolls rotate at the same linear velocity. The control mechanism was a cause of disturbance against the miniaturization of the apparatus.
Therefore, a fixing system using an endless film has been developed for the purposes of: shortening waiting time; saving electric power; and reducing the size of the apparatus. FIG. 2 shows an example of such a fixing system. As shown in FIG. 2, a seamless type endless film 2 is fitted on the outside of a stay 1. A pressure roll 4 is disposed so as to face a heater 3 built in the stay 1. A recording material 6 having a non-fixed toner image 5 temporarily attached thereto is fed in between the endless film 2 and the pressure roll 4. As a result, the non-fixed toner image 5 is melted through the endless film 2 by heat of the heater 3, so that a fixed image 7 is formed. According to this system, the endless film 2 may be made thinner and a heat-conductive filler may be filled in the endless film 2 in order to enhance its heat conductivity. Hence, heat of the heater 3 can be immediately transmitted to the recording material through the endless film 2. Hence, the temperature of the film surface can be immediately raised up to a predetermined value compared with the heat roll fixing system. Accordingly, there is an advantage in that waiting time can be shortened and consumed electric power can be reduced.
As such an endless film for use in such a belt fixing system, there is known a complex tube-like material in which an outer layer of a material made of a heat-resistant resin such as polyimide excellent in heat resistance and mechanical strength is coated with a releasable resin outer layer made of a fluororesin excellent in high-temperature characteristic and toner releasability. There is further known an endless film having film stiffness which is controlled by improvement of heat conductivity at the time of fixing through heat-conductive particles contained in the heat-resistant resin forming the inner layer, or which is controlled by adjustment of monomer components of polyimide (see JP-A-8-80580 and JP-A-7-186162). In those endless films specifically disclosed in those publications as mentioned above, there was however no sufficient consideration of abrasion resistance of the releasable resin outer layer. It was found that fixing failure due to abrasion of the outer layer occurred in fixing under a high-pressure and high-temperature condition with the advance of printing speed in recent years.
On the other hand, in order to improve abrasion resistance of the releasable resin outer layer, it is conceived that a fluororesin having a melting viscosity of not lower than 1xc3x97104 poise and having a relatively high molecular weight is preferable as the resin to be used in the outer layer.
According to the inventors"" experiment, it was however proved that a coating film excellent in abrasion resistance, high in density, and high in hardness could not be obtained when a fluororesin high in molecular weight was merely used. Even in the case where a coating film high in hardness (for example, a coating film having Vickers hardness of not lower than 12 Hv) was obtained, it was also however proved that the effect to improve abrasion resistance of the endless film surface was small when the inner layer of the tube-like material was high in stiffness. These problems were remarkable because the stiffness of the film became high particularly in the case where a large amount of filler powder such as heat-conductive inorganic powder was mixed with the heat-resistant resin layer of the film as shown in the image fixing apparatus in which heating was performed through the film.
On the other hand, when the stiffness of the inner layer of the endless film was made low, the low stiffness was favorable to abrasion. However, a member (not shown) for regulating an edge portion of the endless film so as to control zigzag (meandering) movement of the endless film might be provided for reducing the size of the apparatus. There was another problem that buckling or bending of the film itself and worsening of carrying characteristic might be caused by contact of the film with the member.
It is therefore an object of the present invention to provide an image fixing apparatus using an endless film in which abrasion resistance is largely improved by presence of the high-hardness releasable resin outer layer so that the endless film is provided with practically satisfactory mechanical strength as well as satisfactory abrasion resistance, whereby the image fixing apparatus can obtain a sharp image for a long term; and to provide such an image-fixing endless film for use in the image fixing apparatus.
To achieve the foregoing object, the inventors of the present invention have made various researches on causes of abrasion of the endless film surface and measures against the abrasion in addition to the aforementioned findings. As a result, it has been found that not only the stiffness of the material but also the flexibility of the tube-like material has a large influence on abrasion. It has been further found that satisfactory abrasion resistance can be obtained in cooperation with the effect of the high-hardness releasable resin outer layer when the flexibility of the tube-like material is in a specific range in terms of ring-crash compression load. Thus, the present invention has been accomplished.
That is, according to an aspect of the present invention, there is provided an image fixing apparatus comprising a drive roll, and an endless film facing the drive roll so as to be in external contact with the drive roll to thereby form a nip portion therebetween, wherein: the endless film is a thin tube-like material which includes a heat-resistant resin tube-like inner layer containing heat-conductive particles, and a releasable resin outer layer formed on an outer surface of the heat-resistant resin tube-like inner layer; the releasable resin outer layer has a coating film hardness of not lower than 12 Hv, preferably 12 to 30 Hv, more preferably 12 to 25 Hv, in terms of Vickers hardness; the thin tube-like material has a tensile modulus of elasticity of not smaller than 6000 N/mm2, preferably 6000 to 15000 N/mm21, more preferably 6000 to 10000 N/mm2; and a ring-crash compression load of the tube-like material is not higher than 5.0 g (4900 dyn), preferably 2.0 to 5.0 g, the ring-crash compression load being a load which is given when a tube-like material having a length of 40 mm is compressed vertically so that a vertical diameter (h) of the tube-like material is deformed to be halved (hxe2x80x2=h/2)
In the above image fixing apparatus, preferably, the drive roll is used as a pressure roll; a heat source is provided inside the endless film; and a non-fixed toner image is heated and fixed through the endless film at the nip portion formed between the drive roll and the endless film.
On the other hand, according to another aspect of the present invention, there is provided an image-fixing endless film comprising a thin tube-like material which includes a heat-resistant resin tube-like inner layer containing heat-conductive particles, and a releasable resin outer layer formed on an outer surface of the heat-resistance resin tube-like inner layer, wherein: the releasable resin outer layer has a coating film hardness of not lower than 12 Hv, preferably 12 to 30 Hv, more preferably 12 to 25 Hv, in terms of Vickers hardness; the thin tube-like material has a tensile modulus of elasticity of not smaller than 6000 N/mm21 preferably 6000 to 15000 N/mm2, more preferably 6000 to 10000 N/mm2; and a ring-crash compression load of the tube-like material is not higher than 5.0 g, preferably 2.0 to 5.0 g, the ring-crash compression load being a load which is given when a tube-like material having a length of 40 mm is compressed vertically so that a vertical diameter (h) of the tube-like material is deformed to be halved (hxe2x80x2=h/2).
In the image fixing apparatus according to the present invention, though the tensile modulus of elasticity of the endless film is not smaller than 6000 N/mm2, satisfactory abrasion resistance can be obtained while practically satisfactory mechanical strength is kept, because the ring-crash compression load of the endless film is not higher than 5.0 g. That is, in the belt fixing type image fixing apparatus in which a small-size film is used for the purpose of making the apparatus compact, the flexibility of the tube-like material as a whole is generally reduced when the stiffness of the inner layer of the tube-like material is high. Hence, the contact area of a portion of the tube-like material which is in external contact with the roll is small, so that the contact pressure of the portion becomes high. In such a manner, shearing stress applied on the interface between the tube-like material and the recording material becomes high correspondingly, so that the abrasion of the releasable resin outer layer surface is apt to advance. However, if the ring-crash compression load is not higher than 5.0 g, the flexibility of the tube-like material becomes good, and the shearing stress applied on the interface becomes therefore low. Accordingly, abrasion resistance of the tube-like material can be made good in cooperation with the effect of the outer layer having a Vickers hardness of not lower than 12 Hv. Moreover, though the flexibility is high, carrying characteristic, or the like, is hard to be deteriorated as described above because the tensile modulus of elasticity is not smaller than 6000 N/mm2. As a result, an image fixing apparatus that can obtain a sharp image for a long term can be provided by use of the endless film having practically satisfactory mechanical strength and having satisfactory abrasion resistance. Hence, reduction in size of the image fixing system and increase in speed thereof as described above can be achieved.
When the drive roll is used as a pressure roll and a heat source is provided inside the endless film so that a nip portion is formed between the drive roll and the endless film, and when a non-fixed toner is heat-fixed through the endless film, the effect of the present invention is made particularly remarkable by use of the endless film having abrasion resistance as described above so that an abrasion problem of the film apt to occur due to shearing stress applied on the interface can be solved.
On the other hand, as described above, in the image-fixing endless film according to the present invention, the ring-crash compression load of the endless film is not higher than 5.0 g though the tensile modulus of elasticity thereof is not smaller than 6000 N/mm2. Hence, satisfactory abrasion resistance can be obtained while mechanical strength is kept practically satisfactory in cooperation with the effect of the outer layer having a Vickers hardness of not lower than 12 Hv.